Dimeric intermediates in the Rec recombination pathway of phage lambda have been isolated from an infection of density labeled and light phage and separating the intermediate density DNA by isopycnic centrifugation in the absence of replication. Circular dimers, linear dimers and circles with a linear branch were found under the electron microscope. The cross-over distribution of the structures were determined by partial denaturation mapping and showed some nonuniformity - one associated with a Chi mutation site and the other at the cos site. These preliminary observations are being repeated and will be extended with Chi mutations at different locations of lambda and in different hosts (RecA, RecB, Po1, lig) known to influence Rec recombination in order to understand the role and sequence of operation of these genes.